Bezos first overtook Gates as the richest man the world in October, when his net worth reached $93.8 million.

A 31-year-old Gates became the youngest billionaire in the world in 1987, shortly after Microsoft went public. In 1995, at age 39, he was the richest man in the world with a fortune of $12.9 billion, NPR reports, and held the title on and off for decades.

But there's a simple reason Gates is no longer the richest person alive, and it's not because of Amazon's tremendous growth. Gates made the choice to give much of his money away.

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In 2000, the same year he stepped down as CEO of Microsoft, Gates co-founded the Bill & Melinda Gates Foundation alongside his wife with the mission of improving the quality of life of people around the world. The couple pivoted to focus on philanthropy full time and have since donated $2.9 billion in cash and 700 million Microsoft shares to charitable causes, Bloomberg reports.

The pair plan give away the majority of their wealth throughout their lifetimes and are doing so by thoughtfully tackling major issues around the world.

Gates isn't just giving away his own fortune, either; he's inspiring others to do the same. In 2010, the Gateses teamed up with billionaire friend Warren Buffett to start The Giving Pledge, a movement for ultra-rich individuals to commit to donating at least half of their wealth to charitable causes during their lifetime.

"We have committed the vast majority of our assets to the Bill & Melinda Gates Foundation to help stop preventable deaths such as these, and to tear down other barriers to health and education that prevent people from making the very most of their lives," Gates wrote in his Giving Pledge letter. "Our animating principle is that all lives have equal value."

One of the Foundation'sproudest accomplishments is the work it has done to eradicate polio. It has contributed nearly $3 billion to help eliminate the disease by 2020 and Gates believes the last outbreak could be on the horizon.

"Progress in fighting polio might be one of the world's best-kept secrets in global health," he acknowledged in the foundation's 2017 annual letter. But soon, he hopes, it will be a secret no more. "If things stay stable enough in the conflicted areas, humanity could see its last case of polio this year."

Since 2007, the Gates Foundation has contributed technical and financial resources to accelerate vaccination campaigns, provide routine immunization and polio surveillance partnerships as well as to help fund more effective vaccines.

And the numbers show that its efforts have made a difference. In 1998, the polio virus was present in over 125 countries and paralyzed 350,000 people every year, mostly children. Since then, polio cases have been reduced by 99 percent, saving 13 million young children from paralysis. Today, there are only twelve known cases of the wild polio virus, according to Jay Wenger, who leads the Gates Foundation polio eradication efforts.

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Bill and Melinda Gates are presented with the 2016 Presidential Medal Of Freedom by President Obama at White House on November 22, 2016 in Washington, DC.

Had Gates held onto the assets he's given to the foundation, he'd be worth an estimated $150 billion today, according to Bloomberg, which would give him the record.

He'd rather put that money to good use.

"We have been blessed with good fortune beyond our wildest expectations, and we are profoundly grateful," Bill and Melinda wrote in their Giving Pledge letter. "But just as these gifts are great, so we feel a great responsibility to use them well."

Bezos hasn't yet made a "a major philanthropic mark," as Bloomberg puts it, but some predict that's in the cards for the billionaire. In June, Bezos put out a request for ideas on Twitter, where he said he was "thinking about a philanthropy strategy." He followed up to say there's more to come, but he hasn't revealed what that will be.

Although Bezos hasn't taken a deep dive into charitable giving just yet, he and his wife MacKenzie have made several sizable donations. He's partnered with Mary's Place, a local organization in Seattle to provide shelter for the homeless and gave $1 million of his personal funds.